This invention relates to a thermal anemometer for measuring velocity of a gaseous flow and particularly to an air flow measuring apparatus for measuring the velocity of an air stream.
In many design developments, control systems and operating systems, detecting or measuring the velocity of the flow of a gaseous medium, and particularly air, is required. A known measuring system includes passage of a portion of the air stream through a passageway including a heated element and comparing the temperature of that element with an ambient air temperature sensing element. A dual element thermal measuring device is disclosed, for example, in U.S. Pat. No. 4,537,068 which issued Aug. 27, 1985.
The velocity monitoring apparatus is advantageously constructed as an elongated wand to provide a portable unit which can be used for monitoring of air flow in various locations. Typically, a thin unit includes a tubular housing with the outer end formed with a narrow sensing passageway as machined part. The heated and ambient temperature elements are mounted within the passageway and connected into a sensing circuit by suitable circuitry passing through the tubular member to an outer handle. The sensing passageway must be carefully shaped to provide accurate detection of the air velocity. Accurate and repeatable velocity measurement also requires relatively precise location of the sensing passageway with respect to the path of the air flow. Thus, rotation of the sensing passageway with respect to the air flow can vary the characteristic of the air flow therethrough, resulting in a distorted output from a true value. Generally, the prior art has used the special outer end member formed as a machined metal part to precisely form a rectangular opening within the outer end of the rod unit. With a precisely machined opening and with accurate and careful location of the wand within the air flow, accurate and repeatable of the air velocity can be obtained. Machined parts are costly and an alternative less costly structure would have significant economic value. The inventor has made an alternate low cost construction using a continuous tubular member with openings cut in opposite sides of the outer end of the tube, using a milling machine or the like, to accurately form identical openings to the opposite side of the tube. Although this structure produces a less expensive and reliable sensor, it is also subject to the requirement of accurate positioning with respect to the air flow such that the passageway is essentially parallel to the path of air flow. Turning of the tube in the plane of the air flow or rotation of the tubular member within the plane of the air flow such that the passageway is not precisely parallel to the air flow changes the output readings. Further, the inventor realized that the inside curved wall structure of the opening may create turbulence which interferes with the output readings.